A lithium ion secondary battery has been developed for various uses and there has been a demand for performance suitable for various uses ranging from use in a small-sized mobile device to use in a large-sized battery-powered electric vehicle (BEV) and a hybrid electric vehicle (HEV).
For use in a mobile device, with the progress of small-size and lightweight electronic devices as well as increase in the power consumption due to the diversification of functions, a lithium ion secondary battery having a higher energy density is required.
In particular, in applications for automobiles, such as battery electric vehicles (BEV) and plug-in hybrid electric vehicles (PHEV), a long-term cycle characteristic over 10 years and a large current load characteristic for driving a high-power motor are mainly required, and a high volume energy density is also required for extending a driving range (distance). Since a large-sized lithium ion secondary battery is expensive, reduction in cost is also required.
Generally, a carbon material such as graphite, hard carbon and soft carbon is used as a negative electrode active substance for a lithium ion secondary battery. While hard carbon and soft carbon described in Japanese Patent No. 3653105 (U.S. Pat. No. 5,587,255; Patent Document 1) are excellent in a characteristic with respect to a large current and also have a relatively satisfactory cycle characteristic, the most widely used material is graphite.
Graphite is classified into natural graphite and artificial graphite.
Among those, natural graphite is available at a low cost and has high discharge capacity and electrode filling property due to high degree of graphitization. However, natural graphite has such problems that it has a high specific surface area since the particles have a scale shape, and that it has a significantly low coulomb efficiency at the initial charging and discharging because the electrolyte is decomposed due to highly reactive edge surfaces of graphite, which leading to gas generation. In addition, the cycle characteristics of a battery using natural graphite are not very good. In order to solve those problems, Japanese Patent publication No. 3534391 (U.S. Pat. No. 6,632,569, Patent Document 2) and the like propose a method involving coating carbon on the surface of the natural graphite processed into a spherical shape.
Regarding artificial graphite, there is exemplified a mesocarbon microsphere-graphitized article described in Japanese Patent No. 3126030 (Patent Document 3) and the like.
Graphitized articles made of oil, coal pitch, coke and the like is available at a relatively low cost. However, a satisfactory crystalline needle-shaped coke tends to align in a scale shape. In order to solve this problem, the method described in Japanese patent publication No. 3361510 (European Patent No. 0918040; Patent Document 4) and the like yield results.
JP-A-2003-77534 (Patent Document 5) teaches that excellent high-rate discharge can be achieved by using artificial graphite having highly-developed pores.
WO 2011/049199 (U.S. Pat. No. 8,372,373; Patent Document 6) discloses artificial graphite being excellent in cycle characteristics.
Japanese Patent No. 4945029 (U.S. Pat. No. 7,141,229; Patent Document 7) discloses an artificial graphite negative electrode produced from needle-shaped green coke having a flow configuration texture which is produced with the addition of boron.
Japanese Patent No. 3725662 (Patent Document 8) discloses a method of removing the boron nitride on the surface of the artificial graphite negative electrode manufactured by the addition of boron.